1. Field of the Invention
The present invention relates to high-temperature components of gas turbines, steam turbines and the like, to a Ni-base alloy used in such high-temperature components, and to a method of producing the Ni-base alloy.
2. Description of Related Art
Many high-strength Ni-base alloys have been strengthened by precipitation of γ′ phase (Ni3Al) and/or γ″ phase (Ni3Nb) within each of crystal grains. In order to provide Ni-base forging alloy with high strength and high ductility properties, it is essential to strengthen crystal grains at those grain boundaries as well as internally. Hitherto for the purpose of restraining occurrence of sliding at grain boundaries of crystal grains and of growth of cracks along the grain boundaries, the grain boundaries have been strengthened by causing lumps of carbide of Mo, Ti, Ta and so on to precipitate at the grain boundaries. However, since those carbides serves as starting points of occurrence of cracks due to low cycle fatigue, recently high-strength Ni-base forging alloys have been provided on conditions of low carbon and restrained amount of carbide precipitates. In this case, since the grain boundaries are made smooth, once the cracks occur, those grow quickly to result in fracture.
In a Ni—Fe alloy (Ni-36Fe-16Cr-3Nb-1.7Ti-0.2Al-0.03C) used to make a gas turbine disk, the η phase, which has been known as a detrimental phase in a superalloy, precipitates in a temperature of 800° C. to 900° C. Thus, the Ni—Fe alloy is subjected to a solution heat treatment at a temperature of 982° C. at which the η phase does not precipitate, and subsequently subjected to aging treatment at a temperature of not higher than 750° C. thereby causing the γ′ phase and the γ″ phase to finely precipitate in each of crystal grains without precipitation of the γ phase. Although the thus obtained material exhibits high strength, it has a characteristic that a growth rate of cracks in a high temperature is high because of a low amount of additive carbon for the purpose of restraining occurrence of cracks thereby causing grain boundaries smooth.
There is shown an evaluation of strength of the above alloy in a literature of “Advances in Materials Technology for fossil Power Plants (Proceeding from the Fourth International Conference, 2004, 587.)”, according to which the alloy has been subjected to an aging treatment at a temperature of 840° C. thereby causing the γ phase to precipitate, and subsequently subjected to an aging treatment at a temperature of not higher than 750° C. thereby causing the γ′ phase and the γ″ phase to finely precipitate. While in general, the η phase precipitates in a lamellar state from an initiation point at grain boundaries, the above literature teaches that the η phase makes the grain boundaries to have zigzag features thereby enabling the crack growth rate to be 1/100.
On the other hand, a literature of “Research Report from the Heat-Resistant Material 123 Committee, Vol. 49, No. 57” discloses that high-temperature ductility is improved, and acceleration of creep deformation is restrained by lamellar precipitation of the η phase at grain boundaries.